Sounds like getting a loan on a one-off is just about as hard as getting insurance
for one. In that case, my pessimistic side would say, "You're in deep doodoo", while it's optimistic counterpart would suggest asking the seller if he/she can hold the note (with a substantial down-payment, of course). After all, unless the seller finds a cash buyer, they're going to have the same issues with other potential purchasers.
Even if that scenario solves the purchase part, you'll probably still have to insure the boat (if only to protect the seller), so I strongly suggest you get a firm commitment from an insurer before
you close the deal. There too, the major players only want to insure boats that fall into their make/model/year grid, but I'm hoping that someone else here can help you find one that doesn't.
I was lucky with my "one-off" in that respect. Yes, I had to pay cash for the boat, but I made the purchase contingent on my ability to procure proper insurance
for it. Since one of the sellers was also the local representative of a prominent insurance company, they managed to bend the rules a bit, but that doesn't mean they can't cancel me down the road...
Aside from the obvious lean holder requirements, many marinas
at least) theoretically require proof of liability insurance before you can dock
or moor in their facilities. I say "theoretically" because to date, no one has asked me for such proof.
In any case, I wish you the best in your endeavor, and many years of smooth sailing once you get it sorted out.